Within the field of pressure sensitive adhesives (PSAs), as a result of ongoing technological developments in the coating process, there exists a continuing demand for innovative developments. Within the industry, hotmelt processes with solvent-free coating technology are of growing importance for the production of PSAs, since the environmental impositions are becoming ever greater and the prices of solvent continue to rise. Consequently, solvents are to be eliminated as far as possible from the manufacturing operation for PSA tapes. However, these efforts are very hard to realize for acrylate PSAs, since these adhesives are still prepared in solvents, and implementing the polymerization in bulk is restricted by the excessive viscosity of the polyacrylates at high molecular weights. Furthermore, the gelling of bulk polymerizations is still a major problem. Gelled polyacrylates can no longer be used for coating and are therefore unsuitable as PSAs for adhesive tapes.
Furthermore, ever more exacting requirements are being imposed on adhesives, since for industrial applications there is a need for transparent PSAs which are stable both to weathering and to temperature (high level of cohesion even at relatively high temperatures) and which also exhibit little outgassing. These requirements can only be met by polyacrylates having a high molecular weight.
U.S. Pat. No. 5,073,611 discloses a relatively low molecular weight acrylate hotmelt which can be efficiently concentrated to the hotmelt and which, through a UV crosslinking mechanism, includes a cohesion-enhancing component. Owing to the low average molecular weight, however, the level of cohesion attained at room temperature and at high temperatures is not satisfactory.
Back in EP 0 621 326 B1 a concentration process for solventborne acrylate PSAs was described. In that batchwise process, high molecular weight acrylate PSAs are processed but the solvent is not recycled, so that with this process it is only possible to utilize the general advantages of hotmelt coating. Furthermore, the residence times are too long, resulting in damage to the polymer from the high temperature exposure (molecular weight degradation, gelling).
The object of the invention is therefore to provide a preparation process for hotmelt-processable polyacrylate PSAs which can be used to produce adhesive tapes, the process operating with little solvent and allowing what solvent there is to be recycled.
In the context of this invention it has been recognized that a low-solvent hotmelt process for polyacrylates is only realizable if the major part of the solvent is recycled and is passed back to the polymerization process. Furthermore, this process must also tolerate polyacrylates having a high molecular weight (about 800 000 g/mol), so that a high level of cohesion (shear strength) is achieved as a result of the long polymer chains. The combination of concentration with simultaneous solvent re-use presupposes that the polyacrylate has been polymerized to a high conversion (in order to avoid residual monomers) and that the system contains no resins or other additives which might release volatile constituents and so adversely affect the recycling concept. These requirements can only be met by a two-stage process.
A further critical point for a hotmelt process is the damage to the polymer as a result of temperature and shearing action. Excessive residence times and high shearing forces lead either to polymer degradation or to gelling of the PSA. Both effects are unwanted. The combination of a short residence time and low shearing force causes serious problems, since in order to realize short residence times it is necessary to operate the extruder at a high speed and thus a high specific drive energy. As a result of the high speed, the residence time is reduced but the shearing of the PSA, and hence the potential for polymer damage, is increased.
A further influence is exerted by the vapor pressure of the solvent. In order to ensure an efficient and gentle polyacrylate concentration process, these polymers are preferably prepared in solvents having a high vapor pressure, since by this means it is possible to achieve a higher concentration rate and it is necessary to introduce less specific energy in order to remove the solvent from the polymer.
Surprisingly and unforeseeably for the skilled worker the disadvantages depicted have been removed by the process of the present invention.